The invention relates to regulating systems (automatic control systems incorporating negative-feedback regulation) operative for regulating the position of the lateral edge of a travelling web of constant web width.
In general such a lateral-edge regulating system (automatic control system incorporating negative-feedback regulation) is comprised of one or more sensing devices operative for sensing the position of at least one of the lateral edges of the travelling web. The edge-position-indicating signal generated by the sensing device is applied to the regulator of the system. The regulator determines whether the edge position is within an acceptable range of positions and, if not, in what direction the edge must be shifted to restore the edge to such range. The regulator then furnishes at its output a corrective positioning signal. This corrective positioning signal is applied to a positioning device, which in turn causes the edge to be shifted back into the range of acceptable positions. The positioning device may be comprised, for example, of an hydraulically activated servo positioner operative for effecting axial shifting of the supply roller off of which the web is being pulled. Alternatively, the positioning device can effect a compensatory swinging movement of one or more web transport rollers in such a direction as to cause the sensed edge of the travelling web to move back into the acceptable range of positions.
When negative-feedback regulation is employed for the specific purpose of regulating the position of the lateral edge of a travelling web (such as an elongated web of paper being fed to a rotary printing machine), a problem which we have found to arise involves system-response dead times resulting for the most part unavoidably from certain relative positions which must be, or as a matter of practice are, maintained between the edge-positon-sensing means of the regulating system, on the one hand, and the position correcting means of the system, on the other hand.
In general, the distance between the edge-position-sensing means of the system and the position-correcting means of the system results from the following. In order to avoid the development of undetected edge position deviations intermediate the edge-position sensing means and the web-consuming machine (such as a rotary printing machine), it is customary to locate the edge-position sensing means as little upstream of the consumer machine as possible. Unfortunately, however, in virtually all practical constructions, there is so close to the web-consuming machine not enough room in which to place the position-correcting means of the regulating system. As a result, the position-correcting means is typically located a considerable distance upstream of the edge-position-sensing means of the system.
This considerable distance between the edge-position-sensing means and the position-correcting means of the regulating system tends to result in the following characteristic behaviour of the edge-position-regulating system:
So long as the web edge remains within the preselected range of acceptable positions (the tolerance range), no position-correcting signal is generated. If the web edge shifts in either direction out of the tolerance range, then a position-correcting signal is generated and persists until such time as the edge-position-sensing means detects a return of the web to within the tolerance range. This position-correcting signal is applied to the position-correcting means of the regulating system, causing the web edge to more or less gradually or suddenly be shifted back into the tolerance range. Because the position-correcting means is located a considerable distance upstream of the edge-position-sensing means, it follows that a certain time, the so-called dead time, elapses before a completed position correction performed at the position-correcting means is actually sensed by the edge-position-sensing means.
As a result of this dead time, the position-correcting means will overregulate. Even after is has shifted the web edge back into the tolerance range, it will continue to shift the web edge in such direction until the dead time elapses and the edge-position-sensing means actually detects the restoration of the edge to the tolerance range. This overregulation manifests itself in repeated overshoots of alternate ones of the two limits of the tolerance range. Each overshoot gives rise to a new corrective action in the opposite direction, so that in the steady state of the regulating system the web edge will shift back and forth out of the tolerance range, continually, unless one resorts to measures described below.
If the overregulation is not counteracted it can become extraordinarily large, particularly when edge position deviations occur quickly in a particular system and/or have large magnitudes when they do occur. Essentially, the principal way of reducing the overregulation is to reduce the speed with which the corrective action is performed in response to the detection of an unacceptable edge-position deviation. However, reduction in the speed of the corrective action leads to a greater amount of waste. For example, if the travelling web is an elongated web of paper being fed into a rotary printing machine, a deliberate reduction of the speed with which the corrective action occurs will necessarily result in the improper (e.g., unacceptably centered) printing of an increased number of sheets cut from the travelling web.
For the foregoing reasons, it becomes impossible, as a purely practical matter, to successfully enough make use of three-point regulators. Three-point regulators are bidirectional regulators which have only three states: positive ON, negative ON, and OFF. Moreover, they exhibit a dead band intermediate the positive ON and negative ON states, so that the regulated variable can vary within a certain tolerance range without triggering a regulating (corrective) action. Specifically, three-point regulators cannot be used successfully enough in the context described above when it is desired to have a regulating system which exhibits both a narrow tolerance range and high speed of corrective action. When both these latter two demands are made of the system, resort must be had to regulating systems operating on a totally analog basis or at least incorporating analog components. Analog regulators are more efficient in this regard than three-point regulators because as the magnitude of the sensed edge-position deviation decreases during the course of a corrective action, the speed of the corrective action likewise decreases, thereby reducing the tendency of the system to go into overregulation. However, analog regulating systems, or regulating systems making extensive use of analog components, are considerably more expensive and more susceptible to malfunction than are for example ON-off regulating system components.
The dead times resulting from web travel from the position-correcting means to the edge-position-sensing means can be reduced to a certain extent by correspondingly decreasing the distance between the two means in question. However, the dead times, usually as a matter of principle, cannot be eliminated altogether because, as a result of the corrective shifting of the web edge back into the tolerance range, edge folds often form in the travelling web. Accordingly, a correct measurement of the position of the web edge can only be performed at a location which is far enough upstream to ensure the disappearance of any edge fold which may have formed. Somewhat similarly, if the position of the web edge is corrected by means of transport rollers which are swingable to "steer" the web to the left or right, the corrective swinging movement of the transport rollers will have an effect upon the sensed position of the web edge only after the elapse of a certain web travel time; accordingly, here likewise, as a matter of principle, the position-correcting means and the edge-position-sensing means must be arranged a certain distance from each other, with the result that the dead time cannot be decreased below a certain minimum.
When in this way the minimum dead time is predetermined by the physical and spatial characteristics of the system is question, design of the system such that position corrections are performed at high speed, in order to compensate quickly for position deviations, and/or in order to handle large-magnitude deviations, so as to reduce the number of, for example, improperly printed sheets cut from the web, tends to result in unstable regulation, just as when a very narrow tolerance range is selected.
If the regulating system is to be very accurate and stable, then the system necessarily must be slow in responding to sensed edge position deviations, with the consequence that a large number of sheets to be cut from the web will be improperly printed, for example. Alternatively, one can utilize high-speed corrective action, but then one must live with an unstable overreactive regulating system which, again, will permit a large number of sheets to be improperly printed.
In practice, one usually makes a compromise, by selecting a high speed for the performance of the position-correcting action, but simultaneously therewith selecting the widest tolerance range acceptable. What results is a system which is not always sufficiently accurate in its action, which furthermore permits a considerable waste of web, and which finally requires relatively expensive analog control components to keep the system as stable as possible. As already mentioned, with an analog regulating system for the positioning of the lateral edge of the travelling web, the signal generated by the sensing means of the system is directly proportional to the position of the web, or sometimes is directly proportional to the magnitude of the position deviation. As a result, the speed with which the corrective action is performed is not a constant throughout the corrective action, but instead decreases as the magnitude of the edge-position deviation decreases. Moreover, even these analog regulating systems often go into overregulation, necessitating the use of additional speed-reducing components for reducing the speed of the corrective action even below that which occurs automatically by virtue of the nature of analog regulation.